exception base class

Hello,

for debugging purposes, I am using next base class for my exception
classes:

class ErrorBase
{
  /*- Class invariants : What must be true after each call to a public
   *                     interface
   */
     public:
         explicit ErrorBase( const std::string &txt );
         ErrorBase( const ErrorBase &rhs );
         virtual ~ErrorBase();

         ErrorBase& operator=( const ErrorBase &rhs );

         virtual std::ostream& Print( std::ostream &stream ) const;

     private:

         void CreateMessage( std::ostream &stream ) const;

         virtual void PrintDetails( std::ostream & ) const;

         std::string txt;
};

then I use something like this:

class RandomError : public ErrorBase
{
   //implementation
}

const std::string errorMsg( __FILE__ + __LINE__ + "error message" );
throw RandomError( errorMsg );

I read on lots of places that the exception class should inherit from
the std::exception class and should not use std::string, because it can
throw. This article explains the problem:
http://www.boost.org/community/error_handling.html

Can anyone recommend me a better way to implement the exception
handling, which is going to give the runtime message when the error
class is thrown?

Thanks in advance.

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